· pumped alkali laser (dpal) operates near 770.1 nm, in the high-rotational limit of the o 2...

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Applied Physics BLasers and Optics ISSN 0946-2171Volume 111Number 2 Appl. Phys. B (2013) 111:173-182DOI 10.1007/s00340-012-5243-y

Investigation of atmospheric $$ {\text{O}}_{2}{\text{X}}{^{ 3}}{ \sum_{\text{g}}^{ - }} \,{\text{to}}\,{\text{b}}{^{ 1}}{ \sum_{\text{g}}^{ + }}$$using open-path tunable diode laserabsorption spectroscopyChristopher A. Rice, Kevin C. Gross &Glen P. Perram

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Investigation of atmospheric O2X3P�

g to b1Pþ

g using open-path

tunable diode laser absorption spectroscopy

Christopher A. Rice • Kevin C. Gross •

Glen P. Perram

Received: 22 February 2012 / Revised: 15 August 2012 / Published online: 31 March 2013

� Springer Basel (outside the USA) 2013

Abstract A tunable diode laser absorption spectroscopy

(TDLAS) device fiber coupled to a pair of 12.5 in. tele-

scopes was used to study atmospheric propagation for open

path lengths of 100–1,000 meters. More than 50 rotational

lines in the molecular oxygen A-band O2X3P�

g to b1Pþ

g

transition near 760 nm were observed. Temperatures were

determined from the Boltzmann rotational distribution to

within 1.3 % (less than ±2 K). Oxygen concentration was

obtained from the integrated spectral area of the absorption

features to within 1.6 % (less than ±0.04 9 1018 mole-

cules/cm3). Pressure was determined independently from

the pressure-broadened Voigt lineshapes to within 10 %. A

fourier transform interferometer (FTIR) was also used to

observe the absorption spectra at 1 cm-1 resolution. The

TDLAS approach achieves a minimum observable absor-

bance of 0.2 %, whereas the FTIR instrument is almost 20

times less sensitive. Applications include atmospheric

characterization for high energy laser propagation and

validation of monocular passive raging.

1 Introduction

Atmospheric transmission in the vicinity of the O2

O2X3P�

g to b1Pþ

g absorption feature is important for

remote sensing [1] and military applications [2]. Environ-

mental applications often require the simultaneous

monitoring of multiple atmospheric and pollutant concen-

tration, which has stimulated the development of multi-

plexing and frequency modulation techniques [3]. In

contrast, atmospheric propagation for high energy lasers

requires high spectral resolution and low detection limits

over long paths. The tunable diode laser absorption spec-

troscopy (TDLAS) technique is well developed and desir-

able for such applications [4].

The majority of TDLAS experiments are performed in

multi-path, White or Herriot cells where the temperature

and pressure are readily controlled. A few applications for

open-path instruments include monitoring greenhouse gas

concentrations from agricultural sites with 10 % uncer-

tainties [5], sensing effluents from military cargo aircraft

with path lengths of up to 15 m [6], determining atmo-

spheric constituents onboard commercial aircraft [7],

investigating methane and ethane over 5 and 15 m open

paths [8], and sensing NO2 with telescopic instruments

over path lengths exceeding 160 m [9]. The challenges of

developing TDLAS instruments for paths of 1 km or

greater, where jitter control, turbulence effects, and field

operations become important, are largely unaddressed.

Recently, Arita et al. [10] examined the O2X3P�

g

to b1Pþ

g transition using multi-mode laser absorption

spectroscopy (MUMAS) for a 10-m absorption cell.

Spectra were recorded at temperatures of 300–500 K, and

pressures of 200–760 Torr. Fitting simulated spectra to the

observations yield uncertainties in extracted temperatures

of 8 K and pressures of \14 Torr. Concentration was

reported with 2 % confidence limits [10]. The current

TDLAS results will be compared to this previous MUMAS

work.

C. A. Rice (&) � K. C. Gross � G. P. Perram

Department of Engineering Physics, Air Force

Institute of Technology, 2950 Hobson Way,

Wright-Patterson Air Force Base, OH 45433, USA

e-mail: [email protected]

K. C. Gross


G. P. Perram


123

Appl. Phys. B (2013) 111:173–182

DOI 10.1007/s00340-012-5243-y

Author's personal copy

The oxygen O2X3P�

g to b1Pþ

g transition is of particular

interest for passive ranging. Techniques estimating range

using the depth of spectral absorption features when

viewing a distant bright broadband spectral source is often

referred to as monocular passive ranging (MPR) [11]. The

O2 (X–b) transition is desirable for passive ranging for

several reasons: (1) it has reasonably constant atmospheric

concentration ratio for dry air, (2) is characterized well as a

function of altitude and meteorological conditions, and (3)

is spectrally isolated from other absorbing species.

A demonstration of the MPR technique for a static

rocket motor test using a Bomem MR-254 Fourier trans-

form spectrometer at a range of 2.8 km yielded range

estimates with an accuracy of about 0.5 % (14 m) [12].

More recently, an intensified CCD array coupled to vari-

able band liquid crystal filter was deployed for a ground

test of a static jet engine in afterburner at ranges of

0.35–4.8 km, establishing a range error of 15 % [13]. The

current work validates the MPR FTIR approach with the

TDLAS active instrument. A second application for the

TDLAS instrument involves characterizing molecular and

aerosol absorption and scattering for high-energy lasers

[14]. In particular, the potassium variant of the diode

pumped alkali laser (DPAL) operates near 770.1 nm, in the

high-rotational limit of the O2 (X–b) PQ and PP branches

[15]. Studying the atmospheric effects of high irradiance

(kW/cm2) laser propagation to distant targets will be

greatly aided by the development of this rugged TDLAS

instrument.

2 The TDLAS device

The tunable diode laser absorption spectroscopy instrument

is shown schematically in Fig. 1. A detailed description of

the device configuration and method for collecting spectra

has previously been discussed [16]. The tunable diode is a

New Focus Velocity laser model 6312 with a 100 mWatt

maximum output, a 10-nm tunable range, and less than a

300-kHz linewidth. The laser source near 760 nm can

easily be changed to investigate other spectral regions of

interest with minimal changes to the overall system. After

amplitude modulation at a frequency of 2 kHz, the laser is

fiber coupled and expanded to nearly fill a military grade

12.5 in. RC Optical Ritchey–Chretien transmit telescope.

The laser beam is directed across an open-path and then

received by a second identical telescope. Two Thor Labs

PDA100As are used for the reference and signal detection

and are analyzed by Stanford Research Systems SR850

dual-phase lock-ins and recorded by a National Instruments

USB-6251. Pellicle beam splitters are used to minimize

etalon effects. The reference intensity is measured late in

the optical train (immediately prior to the telescope beam

expansion) and attenuated to balance sent and received

signals. A High Finesse WSU-2 wavemeter is used to

determine the frequency axis and is calibrated with a fiber-

coupled SIOS SL-03 frequency-stabilized HeNe laser to

achieve at better than 10 MHz accuracy. All hardware is

connected to and controlled by a PC using MatLab. During

an experiment, the laser is finely tuned by driving a piezo

to tune over an approximately 0.1 nm range, and then

coarsely tuned to the next spectral region. The process is

repeated for 95 free spectral ranges to cover a 10 nm

spectral region. Calibration of the frequency axis, baseline

removal, and further spectral processing has recently been

reported [16].

System noise is generally dominated by telescope jitter

and atmospheric turbulence over the path, although for

paths of 100 meters, these effects are usually small.

Spectra have been recorded over 100 m to 1 km open-

paths, and the maximum open-path distance is expected to

be approximately 5 km, a limitation of the QuickSet

QPT-130 ruggedized pan and tilt mounts used. Collection

geometries consist of the transmit and receive telescopes

Fig. 1 Diagram of the TDLAS

of system

174 C. A. Rice et al.

123


next to each other separated by 1 m, each viewing a 6-in.

mirror placed at a variety of distances giving total path

lengths from 100 to 1,000 m. 1 km collections took place

outdoor after several inches of snowfall with an average

temperature of -8 �C, a 48 % relative humidity and a

pressure of 989 hPa, during the collection time.

The performance of the active TDLAS system was

compared to a passive fouier transform spectrometer. An

ABB Bomem MR-304 FTIR spectrometer, coupled to a

reflective telescope with a 4.9-mrad field of view and 12 in.

aperture, was placed in between the TDLAS transmit and

receive telescopes and viewed a 700-Watt halogen broad-

band source placed above the FTIR viewed through the

same mirror described above. The path viewed by the

TDLAS and FTIR are nearly the same. This FTIR uses a

silicon (Si) detector with a spectral range of 8,000–

15,000 cm-1 and an indium gallium arsenide (InGaAs)

detector with a spectral range of 6,000–12,000 cm-1 and

spectral resolution of 1 cm-1. The Si detector provided

superior signal-to-noise in the 760 nm spectral region. The

FTIR device collected interferograms at 10 Hz resulting in

spectra with 1 cm-1 resolution. Calibration and post pro-

cessing techniques have previously been described [12].

Two NIST-certified Davis Vantage Pro2 weather sen-

sors were placed near the telescope and mirror. Tempera-

ture, pressure, and relative humidity measurements were

recorded with instrument variations of 1 �C, 2 hPa, and

6 %, respectively.

3 Results

A typical TDLAS spectrum in the vicinity of the molecular

oxygen X3P�

g �b1Pþ

g (v0 = 0, v00 = 0) band is shown in

Fig. 2a, with approximately 95 free spectral ranges and

over a million data points in the full collected spectrum

with only every 25th data point plotted in the figure. The

rotational spectrum of the O2 (X–b) absorption is assigned

as DKDJ K 00ð Þ where K is the total angular momentum

without electron spin, and J represents the total angular

momentum (spin ? rotation). Figure 3 shows the energy

levels involved for the PP(3) and PQ(3) transitions. The

spectrum of Fig. 2 exhibits two of the four rotational

branches DKDJ ¼P P;P Q;R R;R Q, described by Hund’s

case (b) coupling for the magnetic dipole transition [17].

The R-branch lines up to K00 = 27 are observable with the

current apparatus. However, the current analysis is limited

to the P-branch where the rotational spacing is larger.

Only the odd values of K are present due to nuclear spin

statistics. The rotational distribution in the ground state

peaks at K’’ = 9, consistent with a temperature near 300 K.

The long path length of 100 m and lack of instrumental

broadening leads to a large peak absorbance of A =

2.36 ± 0.002, despite the long radiative lifetime of

*11.4 s [18]. The spectral resolution is limited by the

pressure broadened lineshapes and complete spectral iso-

lation is achieved.

The absorbance, A, for each rotational feature is

described by the Beer–Lambert law:

ADK;DJ;DK00 vð Þ ¼ � lnIt vð ÞI0 vð Þ

� �

¼ rDK;DJ;K 00 vð ÞN K 00; J00ð ÞL

ð1Þ

where the natural logarithm of the send, I0, and received, It,

signals is equal to the product of the frequency dependant

absorption cross-section of the specific rational feature,

rDK;DJ;DK00 ; the number density of molecular oxygen in the

specific rotational level, N(K’’,J’’); and the optical path

length, L. I0 and It are initially balanced using an iris on the

I0 detector to allow similar gain settings between both

detectors and their respective lock-ins. The absorption

cross-sections are specified as:

where the degeneracies for the ground and exited states are

gJ00 ¼ 2J00 þ 1ð Þ and gJ0 ¼ 2J0 þ 1ð Þ, and DvL and DvD are

the Lorentzian and Doppler linewidths (FWHM):

DvL ¼296

T

� �n

cair p� psð Þ þ cself ps

� �

DvD � 7:2� 10�7v0 DK;DJ;DK 00ð ÞffiffiffiffiffiT

M

r ð3Þ

where cair and cself are the air and self-broadening

coefficients, p and ps are the total pressure and partial

pressure, n is the coefficient of temperature dependence on

the air-broadened linewidth, all defined by the 2008

HITRAN database [18]. M is the species mass in AMU,

T is the temperature of the atmosphere in Kelvin. The state

specific spontaneous emission coefficients ADK;DJ;DK00 , and

line positions v0 DK;DJ;DK 00ð Þ are also referenced from

HITRAN. The Voigt lineshape gv is area normalized so

that the frequency integral absorption is:

rDK;DJ;DK00 vð Þ ¼ gJ 0

gJ00

ADK;DJ;DK 00k2DK;DJ;DK 00gv v� v0 DK;DJ;DK 00ð Þ;DvL;DvDð Þ

8pð2Þ

Investigation of atmospheric O2 175

123


r0DK;DJ;K 00 ¼

Z1

0

rDK;DJ;DK00 vð Þdv ¼ gJ0

gJ00

ADK;DJ;DK 00k2DK;DJ;DK00

8p

ð4Þ

The Boltzmann distribution specifies the rotational

dependant number density:

NðK 00; J00Þ ¼ ð2J00 þ 1ÞNf

QðTÞ e�EK00 ;J00

kT ð5Þ

where N is the total concentration of oxygen, Q(T) is the

HITRAN rotational partition function that is approximately

equal to hcB=kT using rotational constant B, f is the

isotopic abundance of 16O2, and EK 00;J00 is the rotational

energy for the ground state.

Figure 2a shows a fit of Eq. (1) to each line in the

observed spectrum. All lines in the spectra were fit simul-

taneously with Voigt lineshapes using a nonlinear least

squares method. Each absorption feature was described

using four parameters: line center, v0; integrated absor-

bance, A0 ¼R

AðvÞdv; Doppler width, DvD; and the

Lorentzian width, DvL. Additionally, a small baseline

(A & 0.0019) was observed and a cubic background was

included. The RMS fit residual, 0.0022, is less than 0.1 % of

the peak absorbance, as shown in Fig. 2b. This detection

limit is comparable to that of achieved for controlled

environment multi-path cells without multiplexed detec-

tion. The fit residuals are unstructured and the spectra

exhibit a signal-to-noise ratio of about 150. Figure 4

demonstrates a Voigt fit to a single line, the PP(5), in the

non-physical log-absorbance to demonstrate the quality of

signal. Figure 2c shows the difference between the data and

a simulation using the line-by-line radiative transfer model

(LBLRTM) with average atmospheric conditions reported

by the meteorological instruments (T = 25.1 ± 1 �C, P =

0.977 ± 0.002 atm, N = 4.911 ± 0.16 9 1018 cm-3), while

Fig. 2d is an LBLRTM simulation using the fitted results

(T = 30.2 �C, P = 1.035 ± 0.053 atm, N =4.825 ± 0.04

9 1018 cm-3). Modest improvements resulted from using the

temperature, pressure, and concentration derived from the

spectra rather than the meteorological instruments, but very

small errors in wavelength prevent smaller residuals unless

line centers were included as fit parameters. The fit line

positions differ from the HITRAN data base by 0.00077 ±

0.00093 cm-1, which compares favorably with the waveme-

ter, best accuracy of 0.00033 cm-1.

3.1 Rotational temperature

Temperature is readily determined from the rotational inten-

sity distribution. The integrated absorbance for each feature,

A0DK;DJ;DK00 ¼

gJ0

gJ00

ADK;DJ;DK00k2DK;DJ;DK 00

8p2J00 þ 1ð Þ

Q Tð Þ e�EK00 ;J00

kT NLf

ð6Þ

can be evaluated to achieve a linear dependence on

rotational energy:

lnA0

DK;DJ;DK 00

2J þ 1ð Þk2DK;DJ;DK 00ADK;DJ;K 00

!

¼ LNNLf

8pQ Tð Þ

� �

� EK00;J00

kTð7Þ

Fig. 2 a Undersampled TDLAS spectrum of O2 (X–b) (0,0) band

with rotational assignments, b fit of Eq. (1) using a series of Voigt

lineshapes, c differences between observed spectra an simulation

using weather instrument data, and d difference between simulation

and data using TDLAS derived atmospheric parameters

Fig. 3 Example energy level diagram of O2(X–b) for the PP(3) andPQ(3) lines


123


Independent weighted linear fits of the observed

absorbance for both the PP and PQ branches are illustrated in

Fig. 5. The variance in fit absorbance, A0, as defined by the

95 % confidence bound for the Voigt fit of each absorption

feature, were employed for the weights. The PP and the PQ

branches resulted in temperatures of 303.5 ± 2.0 K and

303.1 ± 1.8 K, respectively. The uncertainty in temperature

is defined by the 95 % confidence bounds in the slope

parameter. The meteorological instruments used during the

collection recorded temperature every minute, and the

average temperature over the duration of collection was

299.5 ± 1 K with confidence derived from the instrument

performance and varied by 3.6 K, during the experiment. It

should also be noted that the temperature sensors were not

in-path, but were located near exterior doors of the building

within 5 m of each end of the path. The average outdoor

temperature was about 290 ± 2 K, and may have contributed

to the lower measured meteorological temperature.

3.2 Concentration

Concentration can be determined from the intercept of

Eq. (7) and the data in Fig. 5 or by the depth of any

rotational feature. Because each line is resolvable,

Fig. 4 A fit of Eq. (1) to thePP(5) rotational line

Fig. 5 Estimations of

temperature using the intensity

distributions of the PP and PQ

branches


123


concentration will be calculated for each line with the

average result giving the final estimate for concentration

using Eq. (5). The resulting concentrations for each tran-

sition in the PP and PQ branches are shown in Fig. 6. The

uncertainty in concentration for a specific line is *0.1 %.

The variance from line to line is greater than 0.8 %.

The best estimate for oxygen concentration is 4.82 ±

0.04 9 1018 molecules/cm3. The weather station provides

a somewhat larger estimate, 4.91 ± 0.16 9 1018 mole-

cules/cm3, with a larger uncertainty. Calculating concen-

tration using Eq. (7) yielded results that agree with the line

by line method giving 4.82 ± 0.05 9 1018 molecules/cm3.

3.3 Pressure

Determining atmospheric pressure from the spectral data is

less precise than concentration and temperature. The high

resolution spectra exist with sufficient signal-to-noise to

distinguish the Lorentzian component of the Voigt line-

shape. Fitting a Voigt lineshape to each spectrally isolated

line provides estimates of the Lorentzian, DvL and the

Doppler, DvD full-width half maximums. The results of the

pressure broadened lineshape, with fixed and unfixed

Doppler widths, along with the expected linewidth using

meteorological data and the HITRAN database are all

illustrated in Fig. 7. There is a strong rotational dependence

to the pressure broadening rates due to the inelastic energy

transfer [19]. Voigt fits to the individual lines, as shown in

Fig. 5, resulted in Lorentzian widths with an average fit

uncertainty of ±0.2 % for each line. Using the distribution

of pressures for individual features, pressure was deter-

mined as 0.975 ± 0.04 atm while the meteorological

instruments reported 0.977 ± 0.001 atm. When tempera-

ture is unconstrained, the Doppler widths are reasonable and

vary by ±5.1 %. This variation is demonstrated in Fig. 8.

By not assuming a fixed Doppler width, a small reduction of

performance in spectral fitting results for pressure broad-

ening and decreased fitted temperature confidence bound-

aries by about 0.2 K. For additional comparison, pressure

broadening widths calculated from data collected using a

Bomem DA-8 fourier transform spectromieter (FTS) [19] is

included in the figure. Spectra in Pope’s data were collected

at 0.032 cm-1 resolution and had a signal-to-noise of about

10 near the deepest rotational lines. The results compare

favorably with the data collected here.

Extending collections to 1 km outdoor open paths is

achieved by keeping the same general geometry but mov-

ing the turning mirror further away from the transmit and

receive telescopes and realigning until the best signal is

obtained. 1 km paths produced deeper absorption features

as demonstrated by the comparison of a 100-m path to a

1-km path in Fig. 9. Implementation of longer path col-

lections make the system more sensitive to telescope jitter

and turbulence shown by the larger standard deviation

about the baseline of DA * 0.012, more than seven times

larger than for the 100 meter path spectrum with the

equivalent system settings. The absorbance between rota-

tional pairs is clearly non-zero over longer paths and these

spectral areas must be excluded from the baseline fitting

process. Some of the deeper absorption features become

nearly opaque over the 300 kHz width of the tuning laser,

and the measurement of absorbance becomes limited by the

system detection noise in the peak of a long path absor-

bance feature.

Fig. 6 Estimation of

concentration from each

rotational line. The lightlydashed line gives the confidence

bounds from the meteorological

equipment, the heavily dashed-dotted line gives the confidence

bounds from the concentration

estimate using the y-intercept

method, and the heavily dashedline gives the confidence 95 %

bounds for concentration using

the fitted areas method


123


4 Monocular passive ranging

The process of viewing bright distant sources and using the

depth of absorption bands to give an estimate of range to the

source is referred to as monocular passive ranging (MPR), and

can be used with a variety of atmospheric species. Oxygen is

an excellent candidate for MPR, being spectrally isolated and

having a stable atmospheric concentration [20]. The TDLAS

instrument was deployed simultaneously with a non-imaging

FTIR spectrometer to validate the MPR technique.

An example FTIR spectrum of the oxygen

O2X3P�

g to b1Pþ

g transition at 1 cm-1 resolution with

4,000 co-added interferograms is demonstrated in Fig. 10a.

The FTIR instrument is fast framing with the ability to

produce 10 interferrograms per second at its maximum

resolution of 1 cm-1. The spectral region with no absorp-

tion features was used to fit a cubic baseline through the X–

b transition to give the spectrum in absorbance as seen in

Fig. 10b. The lower spectral resolution of the FTIR spectra

is apparent in the R-branch, and even P-branch rotational

Fig. 7 Estimation of pressure

from meteorological data and

the HITRAN database

(triangles), from the fitted Voigt

decomposition by fixing the

Doppler width (circles), and by

fitting to the Doppler and

Lorentian width simultaneously

(diamonds)

Fig. 8 Estimation of Doppler

widths derived from (o) NLLS

fitting of the and (D) assuming

T = 298.3 K


123


lines are not fully resolved. Baseline noise is also signifi-

cantly higher than TDLAS spectra.

In order to compare the performance metrics between

the two instruments, statistics concerning the accuracy of

integrated absorbance is compared as a function of col-

lection time. The uncertainty in absorbance from the fully

integrated spectra as a function of the number of scans is

shown in Fig. 11a along with a single TDLAS scan having

an absorbance converges to 5.915 with a standard deviation

of 0.046. The FTIR spectra exhibit an integrated absor-

bance of 5.2–6.2, converging to 5.784 with a standard

deviation of 0.252. The smaller variance in TDLAS

absorbance is shown by dashed horizontal lines in Fig. 11a.

The standard deviation of integrated absorbance as a

function of the number of co-adds in Fig. 11b gives the

confidence bounds found in Fig. 11a for FTIR data points,

while the overall confidence for statistical performance of

FITR integrated absorbance is shown by curved dotted

lines. It should be noted that the data shown here uses both

the P and R branches, but the final TDLAS implementation

can allow for a single branch to be used for integrated

absorbance with similar performance outcomes.

Fig. 9 A comparison of spectra

collected at 100 m and 1 km

Fig. 10 An example spectrum

produced from 4,000 co-added

FTIR scans, before (a) and after

(b) the baseline correction. The

points used for fitting a baseline

to raw FTIR data are shown by

asterisks


123


5 DPAL atmospheric transmission

A final experiment was performed to demonstrate the

application of this TDLAS device for the field studies of

laser transmission. The potassium variant of the diode

pumped alkali laser (DPAL) operates on the D1 line near

12985.185 cm-1 [21] in the high rotational limit of the

O2(X–b) (0,0) P-branch, as shown in Fig. 12. The spectra

were acquired over a 150 meter outdoor path during night-

time conditions and weather periods with low wind. During

the data collection, a Triad Technologies potassium vapor

cell with zero pressure (no buffer gas) was placed in the

optical path just before the detector on the receive tele-

scope. With the cell in place, collections were conducted

with cell temperatures of 65 �C giving an approximate

vapor pressure of 2 9 10-5 mbar. Temperatures were

controlled with a Watlow temperature controller and the

cell was incased in a custom aluminum heat block with

thermocouples placed in bored holes in the aluminum

block within 4 mm of the vapor cell. Figure 12 demon-

strates that the potassium D1 absorption line lies nearly

midway between the P-branch K’’ = 31 and K’’ = 33

(v’’ = 0, v’ = 0) oxygen lines. The LBLRTM simulation

using the TDLAS extracted atmospheric conditions is also

shown. The K DPAL system operates at high pressure,

1–20 atm, depending on the spectral width of the diode

pump lasers. The collision induced broadening and line-

shifts can shift the K D1 line by up to 0.5 cm-1. Also

note, at longer path lengths the O2(X–b) hot bands have

rotational lines closer to the K D1 line and could degrade

laser transmission. Further, characterization of atmo-

spheric transmission for the K, Rb, and Cs variants of the

DPAL is currently in progress using the current TDLAS

apparatus.

Typical spectra collected using the TDLAS technique

compares favorably compared to the MUMAS technique

outlined in Arita et al. [10]. While TDLAS collection times

are longer, the resulting raw spectrum is detailed enough to

easily extrapolate concentration, temperature, and pressure

of observed atmospheric species without extensive post-

processing. Sensitivity of MUMAS is reported to be

0.01 % for 128 averaged spectra, while the TDLAS tech-

nique can detect 0.2 % over a single scan. Measured values

for concentration, temperature, and pressure for MUMAS

collections of the O2(X–b) is reported as ±2 % for each

measurement, while TDLAS measurements are ±0.7, ±0.8

and ±4 %, respectively.

Fig. 11 Demonstration of the

statistical standard deviation of

integrated absorption a as a

function of the number collected

spectra. Statistics from the FTIR

data was used to assign

confidence bounds for

integrated absorption for FTIR

collections b and expected

performance of FTIR

collections as a function of scan

time. Results for integrated

absorption are shown with the

horizontal line with one

standard deviation of confidence

for a consecutively long-

running TDLAS collection are

also shown in (a)

Fig. 12 The location of the potassium D1 line between the K’’ = 31

and 33 lines in the molecular oxygen P-branch


123


6 Conclusions

A system to conduct open-path TDLAS measurements has

been developed to investigate the molecular oxygen A-

band O2X3P�

g to b1Pþ

g transition lines near 760 nm. The

resolution of the system was limited to laser linewidth of

less than 300 kHz and was sufficient to separate Voigt

lineshapes into their Lorentzian and Doppler components.

Analysis of the data has shown that accurate estimates for

temperature, concentration, and pressure can be found.

Temperature was determined to within 1.3 %, concentra-

tion to 1.6 %, and atmospheric pressure to less than 10 %.

System noise is limited by turbulence in the atmospheric

path and telescope jitter. The laser source can easily be

changed to investigate other spectral regions. When com-

pared to collection times using FTIR instruments, signal-

to-noise is similar, however, the TDLAS instrument can

record spectral lineshapes of absorption features. The

TDLAS instrument has been deployed to laser test ranges,

operated at temperatures as low as -10 �C for several days

without performance degradation. Path lengths have been

extended to 1 km with minimum detectable changes in

absorbance of DA = 0.06.

Acknowledgments This work was supported by the High Energy

Laser Joint Technology Office (HEL-JTO) and Air Force Office of

Scientific Research (AFOSR).

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http://staff.science.uva.nl/~tgtiecke/PotassiumProperties.pdf

http://staff.science.uva.nl/~tgtiecke/PotassiumProperties.pdf

· pumped alkali laser (dpal) operates near 770.1 nm, in the high-rotational limit of the o 2...

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